1. Field of the Invention
The present invention relates to an organic electroluminescent display device, and more particularly, to an organic electroluminescent display (OELD) device and a method of driving the same.
2. Discussion of the Related Art
Until recently, display devices have typically used cathode-ray tubes (CRTs). Presently, many efforts and studies are being made to develop various types of flat panel displays, such as liquid crystal display (LCD) devices, plasma display panels (PDPs), field emission displays, and electro-luminescence displays (ELDs), as a substitute for CRTs. Of these flat panel displays, organic electroluminescent display (OELD) devices are self-luminescent display devices. The OELD devices operate at low voltages and have a thin profile. Further, the OELD devices have fast response time, high brightness, and wide viewing angles.
FIG. 1 is a circuit diagram illustrating an OELD device according to the related art, and FIG. 2 is a circuit diagram illustrating another OELD device according to the related art.
Referring to FIGS. 1 and 2, a pixel of the OELD device of FIG. 1 includes first to fourth transistors Tr1 to Tr4 and a capacitor C1. A pixel of the OELD device of FIG. 2 includes first to fourth transistors Tr1 to Tr4, and first and second capacitors C1 and C2. For convenience' sake, the OELD devices of FIGS. 1 and 2 are referred to as first and second OELD devices, respectively.
In each of the first and second OELD devices, the first transistor Tr1 is switched by a gate voltage Vg1 or Vg2 applied to a gate line to write a data voltage Vdata to the pixel in each frame. The second transistor Tr2 functions to sample a threshold voltage Vth of the third transistor Tr3. The third transistor Tr3 functions to supply a driving current IOLED to an organic light emitting diode OLED. The fourth transistor Tr4 functions to supply an initialization voltage Vinit to the pixel.
The configuration of the pixel of each of the first and second OELD devices is provided to cope with a variation of the threshold voltage of the third transistor Tr3 due to a variation of a property of the third transistor Tr3 in operation and/or due to a variation of a manufacturing process.
A first driving voltage Vdd_EL is connected to the organic light emitting diode OLED, and a second driving voltage Vss_EL is connected to the third transistor Tr3.
A method of driving the first and second OELD devices is explained with reference to FIGS. 1 to 3.
FIG. 3 is a timing chart of gate voltages and control voltages to drive the OELD devices according to the related art.
Referring to FIGS. 1 to 3, the fourth transistor Tr4 is turned on per frame F according to an initialization control voltage Init1, Init2, Init3, or Init4, and the initialization voltage Vinit is applied to the gate of the third transistor Tr3. Then, the second transistor Tr2 is turned on according to a control voltage Con1, Con2, Con3, or Con4, and the threshold voltage Vth of the third transistor Tr3 is sampled to the gate of the third transistor Tr3. Then, the first transistor Tr1 is turned on according to the gate voltage Vg1, Vg2, Vg3, or Vg4, and a data voltage Vdata is written into the pixel. The third transistor Tr3 adjusts the driving current IOLED, which flows through the third transistor Tr3, according to an amount of the data voltage Vdata. Accordingly, the driving current IOLED is supplied to the organic light emitting diode OLED, and light is emitted from the organic light emitting diode OLED according to an amount of the driving current IOLED.
The driving current IOLED is expressed as a following formula, IOLED=½*μ*COX*(W/L)*(Vgs−Vth)2. In the formula, μ is a mobility of the third transistor Tr3, COX is a capacitance of the third transistor Tr3, W/L is a ratio of width to length of a channel of the third transistor Tr3, and Vgs is a gate-source voltage between the gate and source of the third transistor Tr3.
The sampled threshold voltage Vth by the sampling operation of the second transistor Tr2 is reflected into the formula, for example, into the gate-source voltage Vgs. Accordingly, the driving current IOLED does not depend on the threshold voltage Vth of the third transistor Tr3. Therefore, the organic light emitting diode OLED emits light irrespectively of the variation of the threshold voltage Vth of the third transistor Tr3. This type of driving method is referred to as a voltage compensation driving method.
However, the voltage compensation driving needs a predetermined time in driving each row line. For example, an initializing period for the initializing operation needs about 3 microseconds (μs), a sampling period for the sampling operation needs about 8 microseconds (μs), and a data writing period for the data writing operation needs about 4 microseconds (μs). A sum of the initializing period, the sampling period and the data writing period is about 15 microseconds (μs). Accordingly, a row line drive period for the voltage compensation driving needs at least about 15 microseconds (μs).
However, when the related art OELD devices have a Full HD (High Definition) resolution, for example, 1900*1080 resolution, and is driven with a frequency of 120 Hertz (Hz), a row line drive period is about ( 1/120)*( 1/1080)=7.7 microseconds (μs). Accordingly, the row line drive period of the related art OELD devices is much less than the row line drive period required for the voltage compensation driving. Accordingly, the related art OELD devices can not normally perform the voltage compensation driving.
As described, since the short row line drive period is allotted in driving the related art OELD devices, there are many limits to displaying images.